The use of CaCO₃ and Ca₃(PO₄)₂ as supports for Fe-Co catalysts for carbon nanotube synthesis: A comparative study

The use of a carbon nanotube (CNT) supported catalyst system enhanced with 'docking stations' along the exterior has been shown to limit the surface mobility of ultra small iron catalyst particles on the CNT surfaces during Fisher-Tropsch synthesis. Generally, the mobility of surface-bound metallic nanoparticles on catalyst supports results in sintering, leading to a subsequent decrease in effectiveness of the catalytic behaviour of the metal nanoparticles over time. A Fe-Co bimetallic mixture (50:50 w/w) was impregnated (5 mass % loading) onto either Ca₃(PO₄)₂ to give < 1% multiwalled CNTs (MWCNTs) or onto CaCO₃ to give high yields of MWCNTs with smoother surfaces. Mixtures of Ca₃(PO₄) ₂-CaCO₃ (0/100 to 100/0) yielded tubes with very rough surfaces with larger diameters and the CNT yield increased as the amount of CaCO₃ in the support mixture was increased. The results suggest that metal-support interactions supersede surface area. We show that the presence of CO₂ generated from the decomposition of CaCO₃ assists in the formation of CNTs and avoids thickening of the CNTs. This was confirmed by performing studies using other supports such as CaO, SiO₂ and Al ₂O₃. These CNTs should be suitable for use as catalysts supports and other industrial applications including polymer composites and ceramics. The support mixture is thus ideal for the large scale production of the CNTs since the supports are cheap and environmentally friendly.